How to harden & temper steel

Nicky SuttonUpdated July 20, 2017

Steel is hardened and tempered so that its properties make it fit for a certain purpose. Cost savings are made when an engineer can create steel with optimal toughness, strength and durability. Steel can be moulded or bent into the required shape and then hardened and tempered to produce the finished object. Most steels with a 0.3 per cent carbon content can be hardened and tempered. When hardening, steel can become brittle and unfit for purpose if it is heated until glowing red and then immediately submersed in cold water. If the heated steel is cooled slowly then the result is soft and more malleable. In the industry, because specific properties are required in different steels for different uses, heating and cooling process are measured and precise. Hardening makes the steel hard but brittle, tempering removes the property of brittleness and makes the steel tough and durable.

Build a brazing hearth by arranging a wall of fire bricks, at least 1 foot high, to the back of your metal table -- this will absorb excess heat during the hardening and tempering process. Place your steel to be heated in front of the wall, on top of one or two more fire bricks. The fire bricks are a safe heat-resistant surface on which to place steel during heating. For example, with a drill bit, heat the tip of it (the useful end) with the gas blow torch until it becomes red hot. The steel should be glowing red.

Pick up the drill bit using the blacksmith's tongs and plunge it into the metal trough of cold water. A hissing sound and steam will result. The tip of the drill bit has been hardened but it is very brittle. It will break easily if used and is not yet fit for purpose.

Clean the head of the drill bit with the emery cloth. Place the drill bit back onto the fire bricks of the brazing hearth. Heat the tip of the drill bit once again with the blow torch; this stage is called tempering. Observe the steel continuously because it changes colour quickly. Watch for the steel changing to blue. This line of blue will travel towards the tip of the drill bit. Turn off the blow torch when the blue line reaches the tip, this shows that the required temperature has been reached.

Pick up the drill bit using the blacksmith's tongs. Place it onto a steel surface, for example, the surface of an anvil. The drill bit should be allowed to cool as the steel surface draws heat away from it. Tempering is complete when the drill bit is cold. It should no longer be brittle but instead tough and durable.

Tip

Be mindful of safety and wear heat-proof gloves and goggles. Clear the area of all flammable objects and general debris before commencing work.